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Curtain of fire. The Galileo spacecraft snapped this picture (top) of what may be a "lava fountain" spurting above a volcano on Jupiter's moon Io. Intense heat within the white blotch blinded the camera's detector, but data from the blotch and nearly simultaneous images from a ground-based telescope helped Galileo scientists illustrate what the 25-kilometer-long molten fountain may look like (bottom). The new image also reveals a kilometer-high cliff curving around the volcano.

Galileo Catches Lava Fountain on Io

SAN FRANCISCO--Astronomers are galvanized by a new image of what may be a curtain of lava spewing above a volcano on Jupiter's moon Io. The picture, snapped by the Galileo spacecraft during its daredevil dive past Io on Thanksgiving and released here today at a meeting of the American Geophysical Union, also reveals a complex, jagged cliff arcing near the volcano. The new features offer the best chance yet for researchers to discern the composition of Io's tortured surface.

Planetary scientists have long known that Io is the most volcanically active body in the solar system, thanks to constant gravitational tugs from Jupiter and its other moons that churn Io's interior. Galileo had previously revealed flows within vast volcanic wounds called calderas. The Galileo team hoped they also might find ribbons of fire called "lava fountains," similar to those that spurt skyward from long narrow gashes in the Kilauea volcano on Hawaii. However, Galileo's camera hadn't yet pointed in the right place at the right time to see one.

That changed during a moment of "dumb luck" on November 25 as Galileo passed within 300 kilometers of Io, says Galileo team member Laszlo Keszthelyi of the University of Arizona, Tucson. The camera was aimed near Io's north pole at a nondescript volcanic feature that the team now calls "Tvashtar," after an Indian sun god. Heat flowing from a 25-kilometer-long stripe within Tvashtar's caldera was so intense that it blinded part of the camera's electronic detector and created a white blotch. However, details of parts of the blotch--especially its wavy top edge and a linear section in the middle--convinced Keszthelyi and his colleagues that they were seeing a curtain of lava at least 1.5 kilometers high. Key support for that scenario came from NASA's Infrared Telescope Facility on Mauna Kea, Hawaii, which happened to detect the Tvashtar hot spot from Earth just 3 hours later. According to planetary scientist John Spencer of Lowell Observatory in Flagstaff, Arizona, Tvashtar was so close to Io's horizon at the time that the telescope would not have seen the hot material unless it was jetting above the surface.

Still, the lava fountain scenario is "pretty speculative," says planetary scientist Gerald Schubert of the University of California, Los Angeles. "Because of the bleeding problem [in the image], it's very uncertain exactly what they're seeing," he says. But Spencer and his colleagues think they may be able to dispel those doubts by determining the 3D shape of the hot spot from earlier observations of Io on the same day, when the telescope had a more direct view.